open your terminal emulator of choice and go to the directory where all the repositories are:
$ cd /etc/yum.repos.d
get the repo file from mullvad (replace "stable" with "beta" in the url if you want the beta repo):
$ sudo wget https://repository.mullvad.net/rpm/stable/mullvad.repo
you can now install mullvad vpn:
$ rpm-ostree install mullvad-vpn
restart your computer so the changes take effect:
$ systemctl reboot
mullvad vpn is now installed on your system, but it needs a system service to work. go to this directory:
$ cd /usr/lib/systemd/system
enable the mullvad daemon service so it autostarts when you open your computer:
$ systemctl enable mullvad-daemon.service
and start the service so you can begin using the vpn:
$ systemctl start mullvad-daemon.service
configure your settings as you wish from there!